1. Field of the Invention
The present invention relates to a method and device for discriminating stillness of a semiconductor exposure apparatus, particularly a step-and-repeat projection alignment and exposure apparatus (stepper) using a reduction optical system.
2. Description of the Prior Art
Particularly, the present invention prevents the computation of the degree of alignment from being adversely affected by the relative movement between a photo-mask and wafer which is produced by an oscillation due to the step-and-repeat motion in a stage. The present invention further prevents a variation in detected signals due to various factors such as the reflectivity of the wafer, the surface condition of a photo-register and other factors from providing an indefinite time required to attain the desired alignment or degrading the accuracy of the alignment.
In general, the semiconductor exposure apparatus (mask aligner) is used for exposing a wafer to the actual element pattern of a mask which functions as an original. Such a semiconductor exposure apparatus utilizes either of (1) an entire surface pattern exposure process characterized by a single shot, or (2) a repetitive (divided) exposure process characterized by a plurality of shots. The whole surface pattern exposure process is used in cases where a wafer is exposed in contact with a mask (contact method), where a wafer is exposed, without using any imaging system, with the wafer being spaced away from a mask by a very small distance (proximity method), where a wafer is maintained spaced away from a mask by a sufficient distance such that they will always be disposed in such a relationship that the pattern on the mask can be imaged on the wafer through a one-to-one imaging system including lenses or mirrors (one-to-one projection method), and so on. The whole surface pattern exposure process exposes the wafer to the actual element pattern on the mask at one time. If it is desired to form a very compactly integrated element, however, the whole surface pattern involves a difficulty in manufacturing a mask, since it has to have, at the unit scale, a very fine pattern to which a wafer is to be exposed.
In view of this problem, the repetitive exposure process has been proposed in which a wafer is exposed at its effective area to a pattern on a mask through a plurality of exposure shots by exposing the wafer to the pattern of the mask through a reduction projection lens system having its magnification smaller than one and moving the wafer and mask relative to each other such that the exposed regions on the wafer will not be overlapped one on another throughout the exposure procedure. In this repetitive exposure process utilizing the reduction projection lens system, the pattern on the mask can be enlarged by a factor of the inverse number of the magnification in the imaging system. Therefore, the above difficulty associated with the manufacture of the mask can be decreased.
The repetitive exposure process utilizes either of an OFF-AXIS alignment system or a TTL ON-AXIS alignment system.
In the off-axis alignment system, the alignment pattern on a wafer is first aligned with an alignment optical system fixed outside of a projection optical system. The wafer is then moved accurately to a location under the projection optical system so that the alignment pattern on the wafer will be aligned indirectly with the alignment pattern on a photo-mask. In the TTL on-axis system, the alignment patterns on a wafer and photo-mask are simultaneously observed through a projection optical system such that the alignment patterns can directly be aligned with each other.
The art of semiconductor elements is being greatly advanced toward a goal at which they are integrated compactly as far as possible and operated at a speed as high as possible. The mask aligner is therefore required to have high resolving power and high degree precision in alignment. But, it is an industrial machine so that high productivity is also desired.
FIGS. 1A, 1B and 1C illustrate three basic flowcharts of the prior art alignment and exposure systems (aligners). FIG. 1A shows the flowchart of an aligner utilized in the contact method, the proximity method, the one-to-one projection method or the like. In such a system, a wafer is processed by a single alignment and a single exposure. FIG. 1B shows the flowchart of a stepper used in the off-axis alignment system in which a wafer is processed through a single alignment and a plurality of exposure steps. A loop including an exposure step is repeatedly carried out several tens of times to a hundred and several tens of times per wafer. To reduce time required in the repetitive exposure loops a stage which can be operated with high accuracy at high speed is provided. Such a stepper has an increased resolving power due to the reduction projection in comparison with the aligner operated as shown in FIG. 1A. The stepper also has an increased accuracy of alignment since the wafer can be compensated in expansion and shrinkage by changing the amount of movement in the step stage. However, since the alignment patterns on the mask and wafer have to be independently positioned by different detection systems, the projected pattern of the mask pattern must be moved to be aligned with the wafer pattern while assuring ultimate correctness of the movement. This results in a larger number of factors of error. It is difficult to increase the alignment in accuracy, suppressing various errors.
A system which can eliminate the above factors of error while assuring the increased resolving power and alignment accuracy is a stepper utilized in the TTL on-axis alignment system (die-by-die alignment system) in which the alignment patterns on the mask and wafer can manually be aligned with each other through a lens system. FIG. 1C shows the flowchart of such a system in which a loop 53 includes steps of alignment, exposure and stage movement. As a result, the system requires an additional time which is equal to the operational time of alignment multiplied by the number of steps in comparison with the stepper in the off-axis alignment system. For the TTL on-axis system stepper, therefore, it is an essential proposition that time is reduced particularly in the alignment operation to assure the necessary throughput in a production machine.
For example, when fifty exposures per wafer are required and if a throughput of 50 wafers/hour is expected, the total time afforded to alignment operation plus exposure plus stage movement should be equal to about one or two seconds. Reduction of time by 0.1 seconds corresponds to the saving of five seconds per wafer. The throughput can be increased by three or four wafers per hour.
The requirement of the reduction of time in alignment is a common theme for all the prior art aligners although the degree of importance is different from one another. FIG. 2 shows a portion of the flowchart shown in FIG. 1B in connection with the alignment operation in greater detail. If this flowchart portion is considered in respect to time required in the alignment operation, there are two significant problems one of which is a delay time t in Step 62. A rest time after the stage has been moved (time through which an oscillation produced in the system can be deemed to be stopped) is variable depending on the amount of movement in the stage or the selection of a stage to be moved, for example. To obviate this, a constant delay time t is provided by adding some safety time to the longest time of the times produced in the above situations. This method, however, eliminates the possibility of speeding-up the process.
The other problem is the number of loops 67 through which detections and movements are repeatedly carried out. Necessarily, as the number of loops 67 is increased, loss of time is increased.
A certain fixed relation exists among four factors, an accuracy .sigma.A in AA (automatic alignment) detection, an accuracy .sigma.S in the movement of the stage, a acceptable-or-not discrimination (tolerance T) meaning an expected accuracy and the number of movements R (the number of re-adjustments of the stage until an alignment is attained). If three of these factors are determined, the remaining factor can substantially be determined. A single AA signal is insufficient to assure the necessary alignment accuracy in the system so that a plurality of the same is taken and the accuracy is improved by averaging all the received signal detection values. If detected signal values of N in number are averaged, the accuracy is improved substantially by a factor of .sqroot.N. If .sigma.S is deemed to be an inherent value in the system and when the tolerance T and the number of movements R are set as expected values, the number N of received signals can necessarily be determined. In the prior art, the number N of received signals was set as an inherent value in the system.
Actually, the variation in received signals is variable depending on different steps in the semiconductor production process or different lots of production in the same process. Accordingly, the variation in an averaged value of N received signals also is variable. As a result, the number of movements is always varied contrary to expectation so that the throughput will be influenced. On the other hand, the accuracy of alignment can be determined based on the signal detection accuracy .sigma.A, the accuracy of movement .sigma.S and the tolerance T. If the detection accuracy .sigma.A is varied, the final accuracy of alignment also is varied.
As will be understood from the foregoing, the alignment operation in the prior art is disadvantageous in that it is impossible to reduce time required in the alignment and yet that the expected throughout and alignment accuracy are unstably and always varied.